Electroluminescent device



Filed Jan. 13, 1958 IIIII ww w INVENTOR DONALD C. LIVINGSTON UnitedStates Patent ELECTROLUMINESCENT DEVICE Donald C. -Livingston, Bayside,N.Y., assignor, by mesne assignments, to Sylvania Electric ProductsInc., Wilmington, Del., a corporation of Delaware Application January13, 1958, Serial No. 708,661

6 Claims. (Cl. 250213) My invention is directed towardelectroluminescent image display devices. Certain types of phosphors,when under the influence of an externally applied electric field, willluminesce, the

intensity of the emitted light being some function of the strength ofthis applied field. Consequently, films or layers formed from suchphosphors can be used as transducers for transforming electrical energyto light energy. These layers, for example, can be formed by dispersingphosphor particles in dielectric media or can be formed from one or morephosphor crystals without a dielectric.

It is known that first and second mutually orthogonal, for example,horizontal and vertical, arrays of parallel, separated electricalconductors can be positioned on each side of such a film or layer toform a crossed-grid structure wherein a portion of the film defined as acell is connected between one horizontal conductor and one verticalconductor. When a suitable electrical potential difference'is appliedbetween any pair of horizontal and vertical conductors, the cellconnected between this pair will luminesce.

The applied potentials can be switched or commutated in such manner asto successively energize each cell in turn, thus producing an effectanalogous to cathode ray tube scanning. Under certain circumstances,however,

"electroluminescent panels of this type will exhibit a spurious efiect,which I call the cross effect. This eflect is produced in the followingmanner. When a positive potential, as for example +V, is applied to aselected horizontal conductor and a more negative potential, as forexample '-V, is applied to a selected vertical con- ..ductor, all otherconductors being held at an intermeand vertical'conductors aremaintained at the same potential, the cells connected between theseunselected conductors do not luminesce.

It is an object of the present invention to improve electroluminescentdevices in such manner that the cross elfect is eliminated.

Y Another object is to provide a new and improved electroluminescentpanel characterized by the absence of the cross elfect.

Still another object is to eliminate the cross effect inelectroluminescent devices of the crossed-grid type by applying a signalto the particular cell to be energized and at the same time applying asmall portion of said signal to all unselected cells.

These and other objects of my invention will either be explained or willbecome apparent hereinafter.

crossed-grid structure.

?,930,897v Patented Mar. 29, 1960 In accordance with the principles ofmy invention, I

array of parallel, separated electrical conductors extending along afirst direction; a second array of parallel, separated electricalconductors extending along a second and non-parallel direction; and anelectroluminescent layer interposed between said arrays and electricallycoupled therebetween. A first impedance network is coupled between afirst terminal and each of the conductors of the first array toestablish a first current path between each first array conductor andthe first terminal; all of the first current paths having substantiallythe same impedance. A second impedance network is coupled between asecond terminal and each of the conductors of the second array toestablish a second current path between each second array conductor andthe second terminal, all of the second current paths havingsubstantially the same impedance.

A signal, the amplitude of which can vary with time,

is impressed across the first and second terminals and,

after being attenuated by an amount determined by the impedance of thefirst and second current paths, is also impressed across each of thecells in the crossed-grid structure. In addition, switching orcommutation means, coupled between the conductors in both arrays and thefirst and second terminals, applies the same signal, substantiallyunattenuated but reversed in phase, to any selected cell and causes thiscell to luminesce to a degree dependent upon the amplitude of theincoming signal.

With the above arrangement, the cross efiect can be substantiallyeliminated. More particularly, when the applied voltages areappropriately adjusted, a spurious luminescent pattern in the shape of across will not appear, the selected cell luminescing "brightly against adim uniform background.

An illustrative embodiment of my invention will now be described indetail with reference to the accompanying drawing.

' Referring to the drawing, a plurality of parallel, separated, verticaltransparent conductors 20, 22, 24, 26 and .28, form a first array andare each coupled through a separate resistor 30 to one end of a variableresistor 32. The other end of this resistor 32 is connected to terminal56. Similarly, a plurality of parallel, separated, horizontal conductors34, 36, 38, 40 and 42, form a second array and are each coupled througha separate resistor 44 to one end of a variable resistor 46. The otherend of the resistor 46 is connected to terminal 58.

An electroluminescent film or layer, not shown, is interposed betweenthe horizontal and vertical conductors and is electrically coupledthereto.

A first amplitude-variable signal is impressed between terminal 56 andgrounded terminal 60. A second signal, opposed in instantaneous polarityor phase to the first signal but otherwise identical thereto, issupplied between terminals 58 and 60. At a selected instant in time,therefore, terminal 56 will be at some positive potential, as

for example +V volts with respect to the potential at terminal 60, whileterminal 58 be at a negative potential of V volts with respect to thepotential at terminal 60.

These voltages, attenuated by a first impedance network composed ofresistors 32 and 30 and coupled between terminal 56 and each of thevertical conductors 22, 24, 26, 28, and by a second impedance networkcomposed of resistors 46 and 44 and coupled between terminal 58 and eachof thehorizontal conductors 34, 36, 38, 40 and 42,v

are impressed across all the cells in the crossed-grid structure; as aresult the same potential, the value of which is determined by theattenuation voltage established by the two impedance networks, isappliedacross all cells in the In addition, a commutator 52 is'connectedbetween terany one of the horizontal conductors, in this exampleconductor 36, to terminal 56. A second commutator 48 is connectedbetween terminal 58 and all of the vertical conductors to couple any oneof the five vertical conductors, in this example 26, to terminal 58.Conse quently, a voltage of 2V is applied across the cell 62 defined bythe intersection of horizontal conductor 36 and vertical conductor 26and this selected cell will luminesce brightly against a dim uniformbackground.

As is well known, all electroluminescent films or layers, whether formedof a crystalline film or as a dielectric suspension of phosphorparticles, exhibit nonlinear brightness-voltage characteristics. Moreparticularly, at low levels of applied voltages the light output fromthese films increases relatively slowly with an incremental increase inapplied voltage, but at higher voltage levels, the light outputincreases much more rapidly with an incremental increase in appliedvoltage. Hence, the operation point can be so selected that thebackground level is substantially dark while the selected cell isrelatively bright.

I have found that when the variable resistors 32 and 46 areappropriately adjusted, each of the vertical conductors, other than 26,can be maintained at a potential of V/3, while all horizontal conductorsother than 36 can be maintained at a voltage of -|V/3. As a consequenee,the voltage across the cell 62 being 2V, the voltage across all othercells in the panel is 2V/3. Thus, the voltage ratio between theenergized cell and the background can be 3:1 as distinguished from knownstructures, wherein a voltage ratio of 2:1 is established between thespot and the spurious cross.

Since the uniform background does not contain a conspicuous geometricalform as does the spurious cross, it tends to dissolve into the ambientillumination. If, as indicated above, the voltage is appropriatelychosen with respect to the non-linear characteristic of theelectroluminescent film, the background can be regarded as esentiallydark.

For proper operation of the circuit, the values of resistors i and 44,which can be but need not be equal, must be large enough to isolate theconductors in each array from each other. Further, the values ofvariable resistors 32 and 45 should be respectively small compared tothose of the respective resistors 30 and 44; otherwise all of theunselected strips will tend to be pulled to the potential of theselected strip.

Further, it will be obvious to those skilled in the art that a singleamplitude-variable signal can be applied to the above arrangementinstead of the two phaseopposed signals as heretofore used, providedthat terminal 60 is eliminated and terminal 58 is grounded. For a givensignal level, however, the voltages across all the cells, establishedwhen both signals are used, will be halved when one signal is employed.

While I have shown and pointed out my invention as applied above, itwill be apparent to thoseskilled in the art that many modifications canbe made within the scope and sphere of my invention.

What is claimed is:

1. In combination, a first array of parallel separated electricalconductors extending along a first direction; a second array of parallelseparated electrical conductors extending along a second andnon-parallel direction; an electroluminescent layer interposed betweensaid arrays and electrically coupledthereto; a first set of resistors,one end of one selected resistor in said first set being connected incommon to one end of all unselected resistors in said first set, theother end of said selected first set resistor being coupled to a firstterminal, the other end of any one of said unselected first setresistors being connected to a corresponding one of the conductors insaid first array; and a second set of resistors, one end of one selectedresistor in said second set being connected in com- ,rnon -to one end ofall unselected resistors in said second 4 set, the other end of saidselected second set resistor being coupled to a second terminal, theother end of any one of said unselected second set of resistors beingconnected to a corresponding one of the conductors in said second array.

2. The combination as set forth in claim 1 wherein both of said selectedresistors are variable resistors.

3. A device responsive to an incoming signal and comprising, incombination, a first array of parallel separated electrical conductorsextending along a first direction; a second array of parallel separatedelectrical conductors extending along a second and non-paralleldirection; an electroluminescent layer interposed between said arraysand electrically coupled thereto; first and second terminals, saidsignal being applied across said terminals; a first impedance networkcoupled between said first terminal and each of the conductors of saidfirst array to establish a first current path between each first arrayconductor and said first terminal, all said current paths havingsubstantially the same impedance; a second impedance network coupledbetween said second terminal and each of the conductors of said secondarray to establish a second current pathbetween each second arrayconductor and said second terminal, all said second current paths havingsubstantially the same impedance, whereby said signal, attenuated by anamount determined by the resistance of said first and second currentpaths, is impressed across each first array conductor-second arrayconductor pair.

4. in combination, a first array of parallel separated electricalconductors extending along a first direction; a second array of parallelseparated electrical conductors extending along a second andnon-parallel direction; an electroluminescent layer interposed betweensaid arrays and electrically coupled thereto; a first set of resistors,one end of each resistor in said first set being connected in common toa first terminal, the other end of any one resistor in said first setbeing connected to a corresponding one of the conductors in said firstarray; a second set of resistors, one end of each resistor in saidsecond set being connected in common to a second terminal, the other endof any one resistor in said second set being connected to acorresponding one of the conductors in said second array; and acommutator coupled between said first and second terminals and theconductors in both arrays to couple a selected second array conductor tosaid first terminal and a selected first array conductor to said secondterminal.

5. A device responsive to first and second push-pull signals andcomprising, in combination, a first array of parallel separatedelectrical conductors extending along a first direction; a second arrayof parallel separated electrical conductors extending along a second andnonparallel direction; an electroluminescent'layer interposed betweensaid arrays and electrically coupled thereto; first, second and thirdterminals, said first signal being applied between said first and secondterminals, said .second signal being applied between said third andsecond terminals; a first impedance network coupled between said firstterminal and each of the conductors of said first array to establish afirst current path between each first conductor and said first terminal,all said first current paths having substantially the same impedance; atsecond impedance network coupled between said third terminal and each ofthe conductors of said second array to establish a second current pathbetween each second conductor and said second terminal, all said secondcurrent paths having substantially the same impedance, whereby a thirdsignal proportional to the difference between said first and secondsignals, attenuated by an amount determined by the resistance of saidfirst and second current paths, is impressed across each first arrayconductorsecond array conductor pair.

6. A device responsive to first and second push-pull signals andcomprising, in combination, a first array of parallel-separatedelectrical conductors extending along aaa eo'r a first direction; asecond array of parallel separated electrical conductors extending alonga second and nonparallel direction; an electroluminescent layerinterposed between said arrays and electrically coupled thereto; first,second and third terminals, said first signal being applied between saidfirst and second terminals, said second signal I being applied betweensaid third and second terminals; a

first impedance network coupled between said first terminal and each ofthe conductors of said first array to establish a first current pathbetween each first conductor and said first terminal, all said firstcurrent paths'having substantially the same impedance; a secondimpedance network coupled between said third terminal and each of theconductors of said second array to establish a second current pathbetween each second conductor and said second terminal, all said secondcurrent paths having substantially the same impedance, whereby a thirdsignal proportional to the difierence between said first and secondsignals, attenuated by an amount determined by the resistance of saidfirst and second current paths is being impressed across each firstarray conductor-second array" conductor pair; and a commutatorv coupledbetween said first and third terminals and the conductors of both arraysto couple a selected first array conductor to said third array conductorpair.

References Cited in the file of this patent UNITED STATES PATENTS2,698,915

Piper Jan. 4, 1955

